It is necessary that all nodes of a distributed digital system, such as a digital switching or transmission system, have precisely synchronized timing so that voice and/or data signals can be transmitted error free between nodes. One way to achieve this timing precision is to transmit timing signals to all nodes from a master signal source so that the timing circuitry at all nodes is synchronized to the same signal. For reliability purposes, it is necessary that the system timing facilities have the capability of maintaining the internode synchronization at all times including circumstances in which the master signal source may fail or become otherwise unavailable.
U.S. Pat. No. 4,736,393 discloses a timing control arrangement the dynamically controls the distribution of timing information in a distributed digital communication system. A reference timing signal is distributed from a reference master node to all other nodes in the system. Each node is connected by links to at least one other node and each node receives timing signals from all the links to which it is connected. Each node selects one of these signals as its timing reference by scanning the various received timing signals to identify the one signal that is received via the shortest path to the master reference node. The path length is given by information specifying the number of intermediate nodes through which the timing signal has traveled from the reference node to reach the receiving node. The path length information is conveyed in a timing priority number (TPN).
FIG. 1 illustrates the system of this patent which is capable of having one master and one designated backup master. In FIG. 1, the nodes are interconnected by the indicated paths with the numbered arrows indicating the TPN number transmitted over the path in each direction. The number adjacent to each node indicates the TPN generated by the node and transmitted on all outgoing links. In FIG. 1, node A is the master node; and node F is the backup master. The network condition portrayed in FIG. 1 indicates that the master node is functioning properly to supply timing signals to the rest of the network.
The system of FIG. 1 functions in the following manner. Master node A generates and transmits a TPN of 1 to node B, as indicated by the arrow pointing in the direction of node B over path 101. Node B receives this TPN of 1, increments it by 1, and tranmits a TPN of 2 over paths 101, 102, and 103 to nodes A, D, and C, respectively. All of the nodes in the system perform similar incrementing and transmitting operations on received TPNs.
As long as master node A is active, the system remains in the stable state illustrated by FIG. 1. However, if link 101 fails, the remaining nodes in the system continue to increment and transmit the lowest received TPN on the ongoing links. For example, when link 101 fails, the lowest TPN that node B receives is a 3. Consequently, node B increments this 3 by 1 and transmit a 4 out on links 101, 102 and 103. The result is that the TPN numbers increase rapidly in magnitude until a predetermined TPN maximum is reached (which in the disclosed system is 30). When the maximum is reached, backup master F detects this fact and begins to source timing information for the system and to transmit a TPN of 1. The system will then arrive at a steady state with the timing information being supplied by backup master F.
When link 101 is repaired, master node A is placed back on line to supply timing information to the remainder of the system. Master node A also transmits a command to backup master node F to stop supplying the timing information for the system illustrated in FIG. 1. Upon receipt of this command, backup master node F ceases to supply the signaling information.
A problem with the aforementioned system is that it only allows for one master and one backup master node which potentially limits the overall reliability of the system if both the master and the backup master nodes should fail. In addition, it is necessary for the master node to transmit a message to the backup master node once the master node resumes supplying the timing information for the system. Whereas this is not a severe limitation, it does mean that the master node does need a record of which node has been designated the backup master node. If for some reason it is necessary to temporarily remove the backup master node from service, it is a problem to designate another node as the backup master node.